XF 2.3 Anyone know to fix app_nav.less css format error

[lutish]

body:before { display: block; content: 'Errors occurred when rendering CSS:\A * public:app_nav.less: variable @_nav-elTransitionSpeed is undefined in file public:app_nav.less in public:app_nav.less (on or near line 1174)\A 1171 | .p-navEl-link\A 1172 | {\A 1173 | float: left;\A 1174| .m-transition(opacity, background; @_nav-elTransitionSpeed);\A 1175 | \A 1176 | &.p-navEl-link--menuTrigger\A 1177 | {'; background: yellow; color: black; padding: 10px; margin: 10px; white-space: pre-wrap; }
1171 | .p-navEl-link
1172 | {
1173 | float: left;
1174| .m-transition(opacity, background; @_nav-elTransitionSpeed);
1175 |
1176 | &.p-navEl-link--menuTrigger
1177 | {
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#0 src/XF/CssRenderer.php(427): XF\CssRenderException::createFromLessException(Object(Less_Exception_Compiler), 'public:app_nav....', '// Note that th...')
#1 src/XF/CssRenderer.php(361): XF\CssRenderer->renderToCss('public:app_nav....', '// Note that th...')
#2 src/XF/CssRenderer.php(269): XF\CssRenderer->renderTemplate('public:app_nav....', NULL)
#3 src/XF/CssRenderer.php(127): XF\CssRenderer->renderTemplates(Array, Array, Array)
#4 test.php(23): XF\CssRenderer->render(Array)
#5 {main}

this theme was default theme for testing only,but get this issue again